In at least one known medical imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient, and after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. Such a medical imaging system typically is referred to as a computed tomography (CT) system.
In known third generation CT systems, the x-ray source and the detector array are located on a rotatable gantry. The gantry rotates around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle are referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, projection data are processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The x-ray source, sometimes referred to as an x-ray tube, typically includes an evacuated glass x-ray envelope containing a cathode and a rotating anode. An induction motor rotates the anode, or target, at a target speed in a well known manner. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode. The x-rays produced by the x-ray tube diverge from the focal spot in a generally conical pattern.
Induction motors are well known and include a stator and a rotor. The stator includes current carrying windings which generate a magnetic field. The rotor is rotatably coupled to the stator, and typically includes a rotor core formed by a plurality of laminations. Rotor bars, or conducting bars, extend through the rotor core and are arranged axially at the outer periphery of the rotor core. A rotor shaft is mounted to the rotor core and extends from the core. One end of the rotor shaft is coupled to the anode so that the anode rotates with the rotor shaft. The rotor shaft axis, of course, is coaxial with the rotor core axis of rotation.
In operation, a supply source impresses an alternating voltage on the stator windings to induce an alternating current in the stator windings. The induced current generates an alternating magnetic field which induces currents in the rotor bars of the rotor. Current flow through the rotor bars results in the generation of magnetic fields. As is well known, the magnetic fields generated by the stator windings and the rotor bars couple and create a torque which causes the rotor to rotate. The stator and rotor operate as a rotating transformer with a secondary (rotor) whose secondary impedance is determined by the cross-sectional area of the rotor bars. The magnitude of the current in the stator windings is affected by the rotor impedance.
In the above described induction motor, the rotor rotates at a speed less than synchronous speed. For example, in a six pole induction motor, the synchronous speed (for sixty hertz operation) is 1200 rpm. The rotor may, however, have an actual speed of 1100 rpm. Such a condition is known as "slip." Factors affecting slip of the rotor include bearing friction, rotor unbalance and target mass.
In a CT system, the rotational speed of the anode preferably is precisely controlled so that an operator can prevent the anode from overheating. If the anode overheats, a scan may have to be interrupted to allow the anode to cool. Interrupting a scan, of course, is highly undesirable. Overheating of the anode can also result in degradation of image quality.
Although controlling the speed of the anode is important, it is difficult to accurately monitor the anode speed due to the high voltage applied across the anode, and the anode rotation within a vacuum. Furthermore, rotor slip inhibits accurate monitoring of anode rotation speed. Specifically, the conditions affecting rotor slip can change while performing a scan, thus causing the rotor and anode speed to change. These changes are difficult to detect and monitor.
It would be desirable to control target rotation speed to avoid overheating the anode and to facilitate providing high quality images. It also would be desirable to provide such control without significantly increasing the cost of the system.